1. Field of Invention
The present invention relates to a trimming apparatus, and more particularly, to a current mode trimming apparatus.
2. Description of Related Art
When manufacturing Integrated Circuits (ICs), electrical characteristics vary in the process of manufacturing for various reasons. This variance in electrical characteristics results in many uncertainties in circuit design. Therefore, in order to reduce the factor of electrical variance, besides continuously pursuing the development of the procedure for manufacturing ICs, another remedy for adjusting varied electrical characteristics is trimming.
Conventional trimming technologies comprise two approaches, i.e. poly fuse and laser cut. In poly fuse, a trimming circuit is coupled to the main IC circuit, and poly-silicon is used in the trimming circuit for connection. When the electrical characteristics of the IC are to be adjusted, the desired value of the electrical characteristics of the IC can be obtained by simply increasing the current to cut the poly-silicon in the trimming circuit and change the circuit structure. In laser cut, the method is similar, except that the poly-silicon is displaced by metal, and the metal is cut by means of a laser instead of current, which can also change the electrical characteristics of the IC.
Referring to FIG. 1A, it shows a block diagram of a conventional trimming circuit. An operational amplifier 110 has an positive input end for receiving a reference voltage VREF, and a negative input end and an output end respectively electrically connected to a drain and a gate of an N-type transistor 120. A source of the transistor 120 is electrically connected to a system voltage VCCA. Resistors 130, 141, 142, and 143 are connected in series between the drain of the N-type transistor 120 and a ground voltage GND. Resistors 141, 142, and 143 respectively have the resistance of 4RΩ, 2RΩ, and RΩ. Pads 151˜153 and fuses 161˜163 are also connected in series with each other between the drain of the N-type transistor 120 and the ground voltage GND. The pads 152 and 153 are further electrically connected to both ends of the resistor 142 respectively. Through the pads 151˜153, any fuse of the fuses 161˜163 is selected to be cut, thereby changing the connection state of the fuses 161˜163. By determining the connection state of the fuses 161˜163, the total resistance Rtot of the resistor strings 130, 141, 142, and 143 can thus be determined. Since desired current Itot=VREF/Rtot, the desired current Itot passing through the transistor 120 can be trimmed by trimming the total resistance Rtot of the resistor strings 130, 141, 142, and 143.
FIG. 1B shows the relation between the total resistance Rtot and the desired current Itot of the conventional trimming circuit in FIG. 1A. Referring to FIG. 1A and FIG. 1B together, the transverse axis of FIG. 1B represents the total resistance Rtot of resistor strings 130, 141, 142, and 143, while the longitudinal axis thereof represents the desired current Itot passing through the transistor 120. It can be clearly seen from FIG. 1B that the trimming result of the conventional trimming circuit in FIG. 1A is not linear, thus the accuracy of the trimming is affected. Furthermore, all fuses 161˜163 are not cut before trimming, and the total resistance Rtot of the resistor strings is at a minimum at this time, thus the conventional trimming circuit must start trimming from the maximum desired current Itot. Therefore, when the desired current Itot is smaller than the predetermined current, the conventional trimming circuit will become inapplicable since it cannot increase the desired current Itot.